Elements of Astronomy , 11 



l\%mean longitude, at ihe commencement of the present 

 century, was 3' 10° 9' 13",0. 



Its velocity varies in different parts of its orbit. Like all 

 the other planets, it is most rapid in its perihelion, and 

 slowest in its aphelion. In the former point it describes 

 an arc of 1° 1' 9'',9 in the course of a day ; and in the latter 

 point it describes an arc of only 37' 11'', 5 in the same 

 period. The mean velocity is 59' 0",7 each day. 



The mean longitude of its perihelion at the commence- 

 ment of the present century was 3' 9" 30' 5",0. But the 

 line of the apsides has a direct sidereal motion of 19' 40",8 

 in a century: which, being referred to the ecliptic, will 

 give it a motion (according to the order of the signs) of 

 r r,9 in a year, or 1° 43' 10',8 in a century. A complete 

 revolution round the line of the apsides is called the ano' 

 malistic yearj and is performed in 363'' 6** 14' a". The 

 perihelion coincided with the vernal equin»)X about the year 

 40S9 before the Christian sera. It coincided with the sum- 

 mer solstice about the year 1230 after Christ: and will 

 coincide with the autumnal equinox about the year 64S3. 

 A complete tropical revolution of the apsides is performed 

 in 20931 years. 



The axis of the Earth is inclined to the plane of the 

 ecliptic in an angle of 23° 27' 57",0: which angle n 

 observed to decrease at the rate of 32", I in a century. But 

 this variation of the angle is confined within certain limits; 

 and cannot exceed 2° 42'. 



The annual intersection of the equator with the ecliptic 

 is not always in the same point : but is retrograde, or con- 

 trary to the order of the signs. Consequently the equinoxial 

 points appear to move forward on the ecliptic : and whence 

 this phenomenon is called the precession of the equinoxes. 

 The quantity of this annual change is 50'', 1 ; or j" 23' SO" 

 in a century. A complete revolution is performed in 25863 

 years. 



The sidereal dc7j, or the time employed by the Earth in 

 rcvolvmg on its axis, is always the same. Its diurnal rota- 

 tion has not varied, the hundredth part of a second, since 

 ihc time of Hipparchus. If the mean astronornical, or 

 civil, day be take.i equal to 24 hours, the duration of the 

 sidereal day will be SS** 3^' 4",1. 



The astronomical, or civil, day is constantly changing. 

 This variation arises from two causes : 1. The unequal mo- 

 lion of the Earth in its orbit; 2. The obliquity of that 

 orbit to the plane of the equator. [The mean and apparent 

 foiar days are never equal, except when the Sun's daily mo- 

 tion 



